Photoman, like most others i'm gonna recommend that you keep the flash on your camera. You're not shooting formals, you're shooting the crowd or the nuptials. Use eTTL at a 1/2 to a stop or so less and let the flash do it's job!
I shoot some weddings and I usually set up strobes in some corner to take non-candid shots of couples or small groups. But when I rove I never take the flash of the camera. I tried using a bracket, and though the light was better it was useless when I turned my camera to portrait as there was always someone standing in the way of the light. For the same reason I don't bounce! I use a Canon 600 which is tall enough to NEVER give me red-eye which is why on-camera is so bad. I shoot lots. I ALWAYS shoot the little groups that are taking selfies, especially if they're with the bride or groom. They are the close friends that count. Use a zoom so you can shoot very fast. Get the +/- adjusted from the start, you probably wont have time to fiddle with it much once you start. Be aware of your angles and shoot low and high to avoid the snap-shot look
Good luck, it's not rocket science. Get good shots and enjoy yourself. Don't forget the extra batteries for the flash.
SS

I have a wedding in two days. It is a practice wedding where the bride is not paying.

I have never done off camera manual flash. Always used on camera flash with TTL.

Even though this is a practice weddding it's a big wedding and I want it to look good for both of us.

The flash transmitter is daunting and very confusing. I'm very intimidated by manual off camera flash.

Do I go for it or practice somewhere else first?

Or maybe try it out a little bit and shoot both ways?

I've done weddings with hot shoe mounted flash, bounced off ceilings and walls. I use manual flash, but you can do just fine with a TTL flash and a little chimping until you get your settings dialed in just right. These are from a wedding I did as a second shooter in 2012 with a Nikon D700, 24-70 and a Sunpak 383.

the key to good shots is to never EVER point a flash directly at a subject from the camera's point of view. Use bounce as much as possible. Forget about the tiny soft boxes, Gary Fong things, and even Rogue Flash Benders, unless you use the extra large and are less than 6 ft away from your subjects. The bigger the light source, the softer and more flattering the light will be on the subjects. Definitely practice before you do this for real. Don't ruin the day for the bride and the families involved. If you have any doubt, hook up with a pro who's got a proven track record with weddings and have them do it, with the provision that you assist - so it will be a learning moment for you and the pro will get all the important shots, and the bride will be more than happy - a win win win situation.

I've done weddings with hot shoe mounted flash, bounced off ceilings and walls. I use manual flash, but you can do just fine with a TTL flash and a little chimping until you get your settings dialed in just right. These are from a wedding I did as a second shooter in 2012 with a Nikon D700, 24-70 and a Sunpak 383.

the key to good shots is to never EVER point a flash directly at a subject from the camera's point of view. Use bounce as much as possible. Forget about the tiny soft boxes, Gary Fong things, and even Rogue Flash Benders, unless you use the extra large and are less than 6 ft away from your subjects. The bigger the light source, the softer and more flattering the light will be on the subjects. Definitely practice before you do this for real. Don't ruin the day for the bride and the families involved. If you have any doubt, hook up with a pro who's got a proven track record with weddings and have them do it, with the provision that you assist - so it will be a learning moment for you and the pro will get all the important shots, and the bride will be more than happy - a win win win situation.

That's great but the problem with manual is that the Inverse Square Law comes into effect on EVERY shot, unless they're posed!
You can't adjust the light power on every shot, you won't get any shots!
Yes, on a stand on TTL could work but you said this is a large wedding. That means LOTS of people and very crowded. Unless someone is gonna be walking around with the light for you, it'll be a tough way to shoot.
Gone are the film days where you only shoot 100 shots.
Make sure you have keeping that flash paired to the transmitter down to a science! In the heat of battle is no time to be pulling out the manual.
My personal experience with a flash bender is not very consistent!
It will certainly be interesting. Let us know how it all goes down and best of luck!!!
SS